Radiation sensor devices are used to detect radiation and provide spatial mapping of radiation intensity in radiation-based imaging systems. Such systems involve detection of incoming radiation, such as X-rays, gamma photons and charged particles, in a wide range of different applications including medical applications. Basically, a radiation source generates a beam in the direction of an object to be examined and a detector measures the intensity of the beam after it has passed through the object. The sensor device detects and measures the information required to produce an image representing the attenuation of the radiation resulting from absorption and scattering by the structure through which the beam traveled.
The sensor device is generally located within the detector unit and comprises a number of radiation sensitive elements (also referred to as pixels or sensor cells) arranged e.g. in a checked pattern in order to provide an appropriate spatial mapping. Such radiation sensitive elements may for instance be adapted for collecting charge or for sensing photons.
Many radiation systems involve radiation sources, such as X-ray tubes or radiation treatment machines, associated with a very high photon flow during the output pulse. This makes it extremely difficult to achieve sufficient resolution and integrating electronic readout is generally required. Integrating readout means that the deposited (integrated) charge on each pixel is collected and monitored at predetermined points of time, preferably when there is no radiation.
One way of accomplishing integrating readout is offered by well-known devices referred to as Charge Coupled Devices (CCDs). A CCD typically comprises a semiconductor surface whereupon islands (pixels), which can hold charge, are provided. The charge of an island can be moved to neighboring islands by changing control voltages surrounding the pixels. During readout the charges are commonly shifted repeatedly in rows towards a charge amplifier, which is located at the edge of the semiconductor structure. The resulting signal is in time consisting of the value of the content of individual pixels. A major problem associated with CCDs and similar devices is the fact that they are very radiation sensitive and the semiconductor surface easily get damaged by X-rays or MeV photons. Such devices are hence not a practically feasible alternative for systems with high charge flow rates.
To avoid radiation damage, newer materials like amorphous silicon and amorphous selenium have been developed. These materials are very radiation resistant and there do exist readout plates of amorphous silicon and selenium in the prior art. It is then generally desirable to have one transistor at each pixel as well as amplifiers arranged for instance at the end of each pixel row, whereby read-out can be realized by reading one row at the time. However, radiation-resistant transistors are extremely difficult to achieve, making the solution with plates of amorphous silicon and selenium rather problematic. Moreover, such plates are very expensive and provide a comparatively slow readout. Previous attempts to accomplish a sufficiently large surface area by means of this technology have not succeeded.
Another way of obtaining integrating readout would be to couple a charge amplifier and/or a multiplexer to every pixel avoiding the very sensitive process of moving charges in a controlled manner on a special surface. Due to complicated electronic structure, implementation of such a solution is generally not practically feasible when the number of pixels grows.
A number of sensor devices, developed for the purpose of providing a continues radiation sensor structure with a large number of pixels, have been proposed in the prior art. One approach is based on joining smaller detectors into large area mosaics. In the European patent EP 0 421 869, for example, a large sensor matrix for capturing images is formed by arranging a number of separate horizontal matrixes partly overlapping in a ladder-like structure. Besides being inflexible and bulky when a large number of sensitive elements, and thus a large number of horizontal matrixes, are needed, the proposed solution excludes realizations with planar or smooth sensor matrix surfaces. U.S. Pat. No. 6,323,475 discloses an alternative to such an approach. It describes a semiconductor imaging device including a detector substrate with a plurality of readout substrates connected thereto, where conductive tracks lead from selected detector positions to offset readout circuit positions.
Attempts have also been made to simplify the electronic circuitry in order to achieve readout for a large array of sensitive elements. U.S. Pat. No. 5,184,018 discloses a device with a common amplifier for each column of sensitive elements, instead of an amplifier for each sensitive element. This is achieved by a comparatively complicated design, where each sensitive element comprises an electric switch and switching lines and read lines are provided at rows and columns, respectively, of the matrix. From the above discussion, it follows that such a solution is comparatively radiation sensitive. Another example is the gamma ray semiconductor detector of U.S. Pat. No. 5,245,191, which aims at providing readout for a larger array of sensitive elements by coupling electrode pads of a semiconductor slab to a particular multiplexer type.
None of the above-cited documents discloses a radiation resistant sensor device presenting a large pixel area. The existing radiation sensor devices are associated with severe drawbacks and limitations and there is a considerable need for an improved radiation sensor device offering efficient readout and improved imaging even at high charge-flow rates.